<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .btn {
            color: #fff;
            font-size: 40px;
            border-radius: 50%;
            background-color: cadetblue;
            width: 50px;
            height: 50px;
            border: 10px solid transparent;
            box-shadow: 0 0 0 1px rgba(0, 0, 0, .3) inset,
                0 .1em .2em -.05em rgba(0, 0, 0, .5);
            background-clip: padding-box;
            position: relative;
            cursor: pointer;
            
            line-height: 30px;
        }

        button::before {
            content: '';
            position: absolute;
            top: -10px;
            right: -10px;
            bottom: -10px;
            left: -8px;
        }
    </style>
</head>

<body>
    <button class="btn">+</button>
</body>

</html>